Wetlands may include marshes, bogs, and swamps. Wetland delineation tends to be controversial because such a determination often pits the interest of environmental protectionists against the interests of landowners. Therefore, standards or guidelines have been created to standardize wetland delineation. These guidelines also attempt to balance the interests of the public and private landowner. According to typical guidelines, whether a particular parcel of land qualifies as a wetland generally depends upon the percentage of the growing season that the surface of the soil is continuously saturated with water.
One example of wetland delineation guidelines includes the Corps of Engineers' Wetlands Delineation Manual of January 1987 (“'87 Manual”). The '87 Manual provides guidelines that may be used to determine whether a particular parcel of land is a wetland. Generally, land qualifies as a wetland if it is continuously saturated to the surface between 5% and 12.5% of the growing season. However, the '87 Manual indicates that many sites are not wetlands despite being continuously saturated to the surface between 5% to 12.5% of the growing season. A delineation in these cases is left to the judgment of the delineator. According to the '87 Manual, sites not continuously saturated at least 5% of the growing season are not wet enough to be considered wetlands.
The '87 Manual provides that, delineators (persons who determine whether a site is a wetland) may consider three parameters, soil characteristics, vegetation, and hydrology, when evaluating whether a site is a wetland. Soil characteristics may be used to determine whether the soils at the site are hydric soils. Hydric soils form under conditions of saturation including flooding that persists long enough to develop anaerobic conditions in the soil. These anaerobic conditions, characteristic of hydric soils, may be observed as color changes in the soils.
The hydrology determination, i.e., the depth to saturation in the soil, is the most controversial determination because a delineator cannot directly observe the hydrologic condition of the ground. Therefore, the delineator must rely on other indicators such as vegetation and soil characteristics to make the hydrology determination. Accurately evaluating the site in this manner requires numerous visits to the site. However, it is not uncommon for a delineator to make a delineation based on only a single visit. After the hydrology determination is made, it may be compared with standard hydrological criteria, such as those found in the '87 Manual.
Digging a pit in the ground and measuring the depth at which water appears in the pit will yield the depth of the water table (the upper boundary of a free groundwater body at atmospheric pressure). However, the depth of the upper boundary of saturated soil. (referred to hereafter as the depth to the saturated soil) is not necessarily equal to the depth of the water table. Capillary action, described in more detail below, may draw water up through the grains of soil to a level above the water table causing saturated soil to occur above the water table. The volume of water located between the depth to the saturated soil and the water table is known as the capillary fringe. Both the depth to the saturated soil and the water table may rise during rainfall and shrink when depletion mechanisms such as drainage, evaporation, and transpiration deplete water from the soil.
Referring FIGS. 1A–1E, the capillary fringe will be explained in greater detail. FIG. 1A depicts a barrel 10 of soil grains 14 and water 12. In FIG. 1A, the water 12 extends above the top surface of the soil grains 14 and water 12. In FIG. 1A, table 16 extends above the surface of the soil, there is no capillary tension. FIG. 1B depicts barrel 10 after water 12 has been depleted from the barrel 10 to the point that the depth to the saturated soil and the water table 16 are at the top of the surface of the soil grains 14. Therefore, there is no capillary fringe.
FIG. 1C depicts barrel 10 after one additional drop of water has been depleted from the barrel depicted in FIG. 1B. Menisci 20 form between soil grains 14 at the surface of the soil. As can be seen in FIG. 1C, the water table 16 has dropped to well below the surface of the soil while the depth to the saturated soil remains at the surface of the soil. Each menisci 20 has water 12 on one side and air on the other. Because the water 12 is attracted to the soil grains 14, the water 12 relentlessly seeks to encompass more soil grains 14. Capillary forces draw the water 12 upward to the surface of the soil forming a zone of negative pressure, known as a capillary fringe 22, between the depth to the saturated soil and the water table 16. The surface of the capillary fringe 22 is formed by menisci 20. The surface of the capillary fringe 22 may also be referred to as the capillary front. The capillary fringe 22 is depicted as a gray area between the water table and the surface of the soil. The capillary fringe 22 will move upward until negative pressure behind it reaches the maximum the menisci 20 can support. In this manner, the capillary forces create a pressure differential across the menisci 20 between the saturated soil and the air above the menisci 20. Above the menisci 20, the pressure is atmospheric. Below the menisci 20, the pressure may be as low as minus 12 inches of water. The negative pressure along the surface of the soil makes a visual observation of the depth to the saturated soil difficult because the surface of the soil may appear dry despite the fact that the soil grains directly underneath the surface grains are fully saturated with water.
FIG. 1D depicts barrel 10 after an additional drop of water has been depleted. In this figure, the water table 16 has fallen to the maximum distance the menisci 20 will support. This is evidenced by the fact that air entry 26 has occurred at the surface of the soil. Because the negative pressure between the menisci 20 and the water table is at its maximum, as the water table drops, it will pull the depth to the saturated soil downward with it. Under the conditions depicted in FIG. 10, the capillary fringe 22 is at its maximum length, which may be as large as approximately 12 inches of water. Again, the negative pressure along the surface of the soil will make the soil appear dry despite the fact that ground remains saturated to the surface of the soil.
Further water depletion from the barrel, as depicted in FIG. 1E will produce a non-saturated condition at the surface of the soil. Under these conditions, the depth to the saturated soil (i.e., depth to the surface of the capillary fringe 22), and the water table 16 will change depths at the same time separated by the full extent of the capillary fringe 22 (up to 12 inches).
Specific yield is the fraction of the saturated soil consisting of water that will drain by gravity when the water table drops. The magnitude of the drop in the depth to the saturated soil from FIG. 1D to FIG. 1E is a function of the specific yield of the soil contained in barrel 10.
In FIGS. 1A through 1E, as water was depleted from the barrel 10, no additional water was added. Of course, this is generally not the case in the field. When water is added, such as by precipitation, the surface of the capillary fringe becomes disturbed when new water fills in the menisci and relaxes the tension between the surface of the capillary fringe and the water table. When the tension is relaxed, the pressure in the area of negative pressure increases to atmospheric and the water table moves upward to the depth to the saturated soil. Under these circumstances, the water table and saturated soils may occur at the same depth which may be above, at, or below the surface of the soil. When the rain ends and the excess surface water has been depleted, the volume of water in the saturated soil decreases restoring the tension between the depth to the saturated soil and the water table.
Referring to FIG. 2, an idealized hydrograph 40 of a wetland can be viewed. Arrow 42 depicts time increasing from the left-hand side to the right-hand side of FIG. 2. In this figure, the capillary fringe 50 spans the distance between the surface of the soil 44 and the water table 48. A period of rain starting at time 54 and ending at time 56 causes the water table 48 to move upward to a location above the surface of the soil 44 creating a volume of water 58 above the surface of the soil 44. At this period of time, the capillary fringe collapses. After the period of rain, water is depleted. Water may be depleted by depletion mechanisms such as drainage, evaporation, and/or transpiration. When sufficient water has depleted, the water table 48 drops in depth and the capillary fringe 50 is reformed. However, the depth to the saturated soil 46 remains at the surface of the soil 44 with the capillary fringe 50 spannning the distance between the surface of the soil 44 and the water table 48. Therefore, FIG. 2 depicts a site where the surface of the soil is continuously saturated with water such as certain types of wetlands.
Referring to FIG. 3, an idealized hydrograph 80 of an upland site may be viewed. Arrow 82 depicts time increasing from the left-hand side to the right-hand side of FIG. 3. Initially, both the water table 88 and the depth to the saturated soil 86 are separated by the capillary fringe 90. Both surfaces continuously drop until the start 94 of a period of rain when the capillary fringe 90 collapses and both the depth to the saturated soil 86 and the water table 88 rise to the surface of the soil 84. As water is added, surface water 98 is appears above the surface of the soil 84. Shortly after the end 96 of the period of rain, water is depleted by depletion mechanisms such as drainage, evaporation, and/or transpiration. As a result of the depletion of water, the water table 88 drops and the capillary fringe 90 is reformed. The capillary fringe 90 maintains the depth to the saturated soil 86 at the surface of the soil 84 only briefly. As the water table 88 drops, the depth to the saturated soil 86 also drops. Therefore, in this idealized depiction of uplands, the depth to the saturated soil reaches the surface of the soil only during periods of precipitation and for a short time thereafter.
In summary, the capillary fringe causes the depth to the saturated soil to be as much a 12 inches above the water table. As mentioned above, the depth of the water table can be determined by the depth at which water occurs in a pit dug into the ground. Such a pit is typically 18 inches deep. However, the depth to the saturated soil cannot be accurately determined in this manner because the membrane of menisci and soil grains forming the surface of the capillary fringe may follow the wall of the pit holding water in the soil and preventing water from entering the pit. Under these circumstances, water will not seep into the pit from the saturated soil so long as the negative pressure of the capillary fringe opposes the seepage of water into the pit. Furthermore, the negative pressure along the pit wall obscures visual observation of the saturated soils (i.e., the soil along the pit wall adjacent to the saturated soils will appear dry despite the fact that the soil behind it is saturated with water). Determining which sites are wetlands is therefore complicated because determining the depth to the saturated soil is difficult.
One method of directly measuring the depth to the saturated soil involves using a tensiometer to measure suction or tension in the soil. This method works by using the tensiometer to determine the depth at which the pressure in the soil changes from atmospheric to below atmospheric (i.e., negative pressure). However, this method has the drawback of requiring blind searching to locate the depth to the saturated soil.
Determining the depth to the saturated soil is useful in determining whether a site qualifies as a wetland. The depth to the saturated soil can be used to determine the duration of continuous saturation at the surface of the soil. The duration of continuous saturation at the surface of the soil can be compared with wetland standards, such as those found in the '87 Manual, to determine whether a particular site should be considered a wetland.
It is clear from the above discussion of the capillary fringe that a determination of the depth of the water table alone is insufficient to determine the depth to the saturated soil. Therefore, a need exists for a more accurate method of determining the depth to the saturated soil in the ground. Further, a need exists for a system capable of performing such a method.